Telephone systems in the United States provide central offices for receiving signals from calling telephones within a particular radius such as one (1) to two (2) miles from the central office and for transmitting telephone signals to such telephones. The telephone signals from a calling telephone are then transmitted through long distances from such central office. The telephone signals then pass to a receiving telephone through a central office within a radius of one (1) mile to two (2) miles from such central office.
The telephone signals are transmitted long distances between central offices through optical fibers which have replaced other media previously provided for such purposes. The optical fibers have certain distinctive advantages over the lines previously provided. They allow a significantly increased number of signals from different telephones to be transmitted at the same time through the optical fibers. They pass the digitally-encoded signals with a higher accuracy than other media.
Various systems have been adopted to carry digitally-encoded signals for telephone, video, and data services. One of such systems now being adopted is designated as asynchronous transfer mode (ATM). This system is advantageous because it recognizes that generally signals travel in only one direction at any one time between a calling subscriber and a receiving subscriber. The system preserves bandwidth in the other direction so that a maximum number of different messages can be transmitted in such direction.
In the prior art, when passing data cells from a control memory at a first station to a host through a system bus and then from the host to a recipient, the host remained on the system bus during all of such transfer. This was disadvantageous because the recipient received information from the host only intermittently. During the time that the recipient did not receive data cells from the host, the system bus was still connected to the host so that the system bus could not be used to transfer data cells for other purposes. As a practical matter, the system bus was tied to the host about seventy percent (70%) of the time. This allowed the system bus relatively little time to perform other functions.